How to set Access-Control-Allow-Headers header in node-http-proxy

Question

I am using the coinbase-pro library to make post request to the coinbase sandbox api through a form on localhost. I am trying to use node-http-proxy to get around a CORS error with no success. Ive been banging my head against the wall for a while on this, any help would be appreciated.

const express = require("express");
const httpProxy = require("http-proxy");

const port = 5050;

const app = express();
const proxy = httpProxy.createProxyServer({});

app.use(function(req, res) {
  delete req.headers["origin"];
  delete req.headers["referer"];
  delete req.headers["host"];


  res.setHeader("Access-Control-Allow-Origin", "*");
  res.setHeader(
    "Access-Control-Allow-Headers",
    "Content-type, cb-access-key,cb-access-passphrase,cb-access-sign,cb-access-timestamp"
  );
  res.setHeader(
    "Access-Control-Allow-Methods",
    "GET,POST,PUT,DELETE,OPTIONS"
  );

  const apiURL = 'https://api-public.sandbox.pro.coinbase.com'
  proxy.web(req, res, { target: apiURL });
});

app.listen(port, () =>
  console.log("Started proxy on port", port)
);

error: Access to fetch at 'http://localhost:5050/orders' from origin 'http://localhost:3000' has been blocked by CORS policy: Request header field cb-access-passphrase is not allowed by Access-Control-Allow-Headers in preflight response.

Answer 1

The answer is here:

I think modifying the proxy response header is not covered in the current doc.

proxy.on('proxyRes', function(proxyRes, req, res) {
  console.log('Raw [target] response', JSON.stringify(proxyRes.headers, true, 2));


  proxyRes.headers['x-reverse-proxy'] = "custom-proxy";
  proxyRes.headers['cache-control'] = "max-age=10000";

  console.log('Updated [proxied] response', JSON.stringify(proxyRes.headers, true, 2));

  // Do not use res.setHeader as they won't override headers that are already defined in proxyRes
  // res.setHeader('cache-control', 'max-age=10000');
  // res.setHeader('x-reverse-proxy', 'custom-proxy');

});

The key is to use proxyRes inside "proxyRes" event like proxyRes.headers[key] = value instead of relying on res.setHeader(key, value) as res.setHeader does not work when key is already exists among the proxy target response headers.